The present invention relates to an optical output stabilizing device for generating a stabilized light output power from a light source, such as a semiconductor device, a discharge tube or the like, having a non-linear light power characteristic with respect to a drive current (voltage), by means of automatically controlling the quantity of light output power to a desired level.
The light source such as a semiconductor device, a discharge tube or the like has a non-linear light power characteristics with respect to the drive current (voltage) for the light source. The non-linear characteristics of the light output power is to be understood to mean a curved line characteristics which show zero or very weak light output power at very small drive current (voltage) and an abruptly increased light output power as the drive current (voltage) is increased. In order to obtain stabilized light output power from the light source having such a non-linear characteristics of the light output power, it is necessary to perform the stabilization of the light output power by controlling the drive current (voltage) suitably before and after a knee point of the curved line. As a typical light source having the non-linear characteristic of the light output power, a semiconductor laser element is described by way of example with reference to the drawings, corresponding elements having been given the same reference numerals.
As shown in FIG. 1, the semiconductor laser element exhibits a non-linear characteristic of the light output power having a curved line portion with respect to a drive current 1. The knee point is, generally, defined by a threshold current I.sub.th and the slope of a steep rising portion after the knee point is defined by a differential quantum efficiency .eta.. The light power characteristics of such a semiconductor laser element exhibits an increase of the threshold current I.sub.th and a decrease of the differential quantum efficiency .eta. due to an increase in ambient temperature of the laser element or deterioration of the element. Such a variation of the light power characteristic is shown in FIG. 1 by C.sub.1 and C.sub.2. The effect of the temperature dependency or the deterioration dependency on such a light power characteristic is remarkable when the semiconductor laser element is driven by a pulse current to obtain a pulsatory light power. FIG. 1 also shows such an effect by light powers P.sub.1 and P.sub.2. This is the case that the semiconductor laser element is used as a light source of an optical memory device, the readout of the stored information is performed by a weak continuous oscillation mode of the laser element caused by a DC current I.sub.R, and the writing or erasing of information is performed by a strong pulse oscillation mode caused by a pulse current I.sub.W. As is seen from FIG. 1, when the light power characteristic is changed from C.sub.1 to C.sub.2 due to the deterioration or temperature rise of the laser element, the continuous oscillation mode (output component corresponding to the DC current I.sub.R) for readout can not often be obtained.
FIG. 2 shows a typical conventional light power stabilizing device for obtaining a stabilized light output power from a light source having non-linear characteristic (also called as Automatic Power Controller). As seen from FIG. 2, a part of light power generated by a semiconductor laser element 1 is detected by a photo detector 2. The output of the detector 2 is supplied through an amplifier 3 to a window comparator 4 which compares the input signal with previously set upper and lower levels to generate output signals having inverse polarity. These output signals are supplied to first input of an AND gate circit 5 which receives at its second input clock signals of a clock pulse generator 6 and supplies its output to a digital up/down counter 7. The output of the counter 7 is supplied through a D/A converter 8 to an perational amplifier 10 having a feedback resistor 11. The output of the amplifier 10 is supplied to a base of a transistor 13 serving as a control means for the element 1 with a series resistor 12, thereby feedback-controlling the device current of the semiconductor laser element 1. In such conventional light power stabilizing device, the control of driving current over whole setting range of the laser element is performed by the window comparator 4, the up/down counter 7 and the D/A converter 8, so that the control precision is limited by the dead zone and the response speed of respective components and the bit number. As shown in FIG. 1, for example, when the semiconductor laser element is oscillated at the pulse oscillation mode (corresponding to I.sub.W component) together with the continuous oscillation mode (corresponding to I.sub.R component), the driving current of the laser element must be set within the range of 0 to 80 mA in order to obtain a pulse output of 3 mW, since the maximum driving current of about 80 mA including compensation of output decrease due to temperature rise must be necessary. When the D/A converter 8 has 8 bit length and two digits are allotted as a precision (resolution) by taking the dead zone of the window comparator 4 into consideration, the resolution (precision) for the driving current setting range becomes 0.6 mA of current value so that precision in light power control becomes 0.2 mW in the case of the laser element having the differential quantum efficiency .eta. of 0.3 mW/mA. That is, only precision of about 7% can be obtained for the maximum power of 3 mW so that such a laser element can not be utilized to the application requring high precision.